Laser printing devices that employ a hologon film scanner to generate a scanning laser beam are well known in the art. The hologon film scanner typically includes a hologon disc containing a number of facets. A laser beam is directed to the hologon disc as the disc is rotated by a drive motor. The laser beam is deflected as it passes through the facets of the rotating hologon disc and the deflected beam is used to scan a line on a photoreceptor, such as a photosensitive drum or belt utilized in some electrostatic printing devices or a photographic film in a film scanner printing device. The power intensity of the laser beam can be modulated to create either a continuous tone image or a number of discrete image pixels.
There is a problem, however, associated with the above-described laser printing devices, namely, print quality can be seriously degraded due to line-to-line and intraline fluctuations in the power intensity of the scanned laser beam. The power intensity fluctuations are caused by facet-to-facet and intrafacet variations in the light transmission efficiency of the hologon disc. The fluctuations in light transmission efficiency, and the problems associated therewith, can be minimized if the hologon disc is manufactured to exacting tolerances. The manufacturing precision required to eliminate the above-described power intensity fluctuations, however, is very expensive and greatly increases the overall cost of a hologon scanner.
In view of the above, it is an object of the invention to provide a method and apparatus for detecting and correcting facet-to-facet and intrafacet variations in the light transmission efficiency of a hologon scanner, thereby improving printing quality while simultaneously lowering the manufacturing expense of the hologon scanner. Other objects and advantages of the invention will become apparent after further study of the detailed description of the preferred embodiments of the invention provided below.